There is an ever increasing demand for data communication networks to be capable of efficiently handling vastly different types of data traffic ranging from low speed data and voice to full motion video. Time Division Multiple Access (TDMA) offers a comparably simple, inexpensive solution to the challenge of sharing a common data communication network among multiple users. The efficiency with which TDMA combines multiple data streams is commonly measured by the ratio of the achievable aggregate data rate to the maximum single-user data rate. Thus, TDMA is most efficient if network data is transferred at an effective rate equal to the maximum possible node-to-node transfer rate.
The challenge of TDMA algorithm design is to achieve high transfer efficiency with a minimum of processing complexity/cost, while satisfying the increasingly tight constraints on the transfer of time-sensitive data, such as voice or video. One constraint is that each network link offers a finite data transfer capacity, i.e. a maximum rate at which data can move from one node to the next. Another constraint is that the data traffic generated by individual nodes can vary significantly over time, as each node evolves through multiple modes of operation. Since the rate at which traffic can be transferred between nodes is limited, there may be some aggregate rate of nodal traffic which cannot be accommodated. In an ideal network, this limit would equal the link's maximum data transfer capacity, but in practical networks, efficiency seldom approaches that limit. TDMA efficiency has declined as modern networks have experienced an increase in the number of users (or nodes) exhibiting time-varying data rates. Furthermore, efficiency has declined due to an expansion in the range of data rates which must be multiplexed together, while imposing strict constraints on timing-sensitive data traffic.
One type of TDMA transmission technique utilizes a circuit transmission protocol. In circuit transmission protocol, a network data stream is made up of frames which are subdivided into time slots. Corresponding time slots in each frame are allocated to specific connections between network nodes within a network. For example, one time slot in each frame may be allocated to one specific connection and a second time slot in each frame may be allocated to a second connection, etc. Each frame also includes a time slot which contains transmission overhead information including frame synchronization words and control words. Circuit transmission protocol may also be extended into multiple bit rate services by allocating multiple slots in each frame to high bandwidth services.
Circuit transmission protocol can efficiently transmit data when the data rate is not changing, however, network users are more frequently switching between voice, data, and video services during a single connection or transmitting semi-bursty data generated by compression algorithms. This results in continuously changing data rates and bursty data. Conventional circuit transmission protocols are not effective in this type of environment because the network can only transmit the data at a bit rate determined by the number of slots allocated to the connection. This lack of flexibility results in a loss of data during transmission when the data traffic is exhibiting changing data rates or is bursty.
In contrast with circuit transmission protocols, a packet transmission protocol transmits data in discrete blocks or packets, with each packet having an address header at the front thereof. At network controllers, packets are routed from a specific input line to a specific output line based on address information contained in the packet header. In this way data packets can be routed from a particular subscriber location, through a data communication network, to another subscriber location, and no pre-established connection defines a fixed rate. Thus the packet transmission protocol is suited for wideband data communication services.
A type of packet transmission protocol known as Asynchronous Time Division Multiplexing (ATDM) is particularly suited for use in connection with bursty data traffic. ATDM uses channel identifiers with actual data to allow on-demand multiplexing of data from subscriber terminals. ATDM is bit rate flexible since the appearance of packets can be asynchronous, as in the case of bursty traffic, and can support sharing between voice, video, and data at a variety of transmission rates.
Another type of packet transmission protocol uses a flexible network transport system that is capable of handling both circuit and packet data. This approach combines conventional circuit transmission protocols and packet transmission protocols. To accomplish this, each payload frame of a network data stream can accommodate either a data packet or a time slot from a circuit transmission stream. Before a slot from a circuit transmission stream can be inserted into the payload field, though, it must first be converted into a packet-like form with a header at its front.
The previously discussed packet transmission protocols share a single flaw which can severely constrain network efficiency. In responding to the random character of much of network traffic, packet transmission protocols impose an unnecessary and wasteful overhead on all network traffic thereby decreasing the overall data transfer efficiency of the network.